The goal of this proposal is to define the genetic basis of ankylosing spondylitis (AS) susceptibility. Together with the other groups (Oxford and Paris, France), who also have completed genome scans of AS, we have completed a meta-analysis by combining 488 pedigrees and 589 affected sib pairs (ASPs), where not only were some previously described chromosomal regions confirmed (on 6p, 6q, 10q, 16q) but also new regions emerged not noted previously (on 1q, 5q). Thus, this project is centered around four hypotheses; 1) that the MHC contribution to the genetic basis of AS, though primarily provided by HLA-B27, is augmented by other MHC influences that require novel approaches to elucidate; 2) that the contribution of non-MHC genes, though vital, is sufficiently small to require large sample sizes and confirmation in independent cohorts including different ethnic groups; 3) that these genetic factors interact with each other in a complex manner to influence disease susceptibility; and 4) that rigid and consistent phenotypic characterization is crucial to the success of any genetic study, particularly that of AS. The specific aims of this project are, therefore: 1) To characterize the MHC contribution to AS susceptibility by using both family based and case-control studies and by examining haplotype blocks spanning the relevant HLA genes; 2. To validate the AS susceptibility regions implicated by the meta-analysis by conducting a genome-scan in a larger collection of pedigrees where consistent criteria of diagnosis (modified New York Criteria with available radiographs) are enforced; 3. To examine 40 potential candidate genes in regions identified in our genomewide scans by examining SNP's and haplotypes of these genes in North American AS families and in unrelated cases and controls, confirmed in British AS families from the same ethnic background and further in Chinese and Mexican families; 4. To investigate interaction of candidate regions/genes identified in Specific Aim 3 with other MHC and non MHC genes implicated in the risk for AS.